Magnesium oxide for annealing separator, and grain-oriented electromagnetic steel sheet

ABSTRACT

An object of the present invention is to provide magnesium oxide for an annealing separator which is useful for obtaining grain-oriented electromagnetic steel sheets with excellent magnetic properties and insulating properties. To resolve the above object, an aspect of the present invention resides in magnesium oxide for an annealing separator having a Blaine specific surface area of 2.5×10 3  to 7.0×10 3  m 2 ·kg −1  and CAA of 50 to 170 seconds.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a National Phase Patent Application and claims priority to and the benefit of International Application Number PCT/JP2017/010690, filed on Mar. 16, 2017, which claims priority to and the benefit of Japanese Patent Application Number 2016-067692, filed Mar. 30, 2016, the entire contents of all of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to magnesium oxide for an annealing separator and to a grain-oriented electromagnetic steel sheet.

BACKGROUND ART

Grain-oriented electromagnetic steel sheets used for transformers and generators are generally manufactured by hot-rolling silicon steel containing about 3% silicon (Si), cold-rolling the steel sheet to a final thickness, and decarburization-annealing and finish-annealing the steel sheet. In the decarburization-annealing (primary recrystallization annealing), a silicon dioxide film is formed on the steel sheet surface, slurry containing magnesium oxide for an annealing separator is applied onto the surface and is dried, and the steel sheet is coiled and is subjected to finish-annealing in which silicon dioxide (SiO₂) reacts with magnesium oxide (MgO) to form a forsterite (Mg₂SiO₄) film on the surface of the steel sheet. This forsterite film imparts tension to the surface of the steel sheet, reduces the iron loss to enhance magnetic properties, and also gives insulating properties to the steel sheet.

To enhance the characteristics of grain-oriented electromagnetic steel sheets, trace components that are contained in magnesium oxide for an annealing separator have been studied. Some trace components that are added in controlled amounts to magnesium oxide for an annealing separator are calcium oxide (CaO), boron (B), sulfite (SO₃), fluorine (F) and chlorine (Cl). Further, attempts to investigate not only the contents of trace components but also the structures of compounds containing trace component elements in magnesium oxide for an annealing separator have been made.

For example, Patent Literature 1 discloses magnesium oxide for an annealing separator having specific contents of CaO and B. Further, Patent Literature 2 discloses magnesium oxide for an annealing separator which contains specific amounts of chlorides such as of Mg and Ca and has a specific ratio of B to the chlorides. Patent Literature 3 and Patent Literature 4 disclose magnesium oxide for an annealing separator which have specific contents of CaO, SO₃, halogen and B in the magnesium oxide for an annealing separator. There are researches which specify other properties of magnesium oxide for an annealing separator. For example, Patent Literature 5 discloses magnesium oxide for an annealing separator having many controlled physical properties including the contents of CaO, CO₂, SO₃, K, Na and B.

Patent Literature 6 discloses a method for manufacturing a grain-oriented electromagnetic steel sheet which uses magnesium oxide having specific contents of Cl and SO₃. Patent Literature 7 discloses an annealing separator for a grain-oriented electromagnetic steel sheet which has specific F and Cl contents and specific properties.

Some inventions have been made which focus attention on, besides trace components, the activity based on the reaction rate of magnesium oxide particles with an acid, namely, the citric acid activity (CAA). The CAA is expressed as the time required for a 0.4 N aqueous citric acid solution containing phenolphthalein as an indicator to be neutralized when it is mixed with a final reaction equivalent of magnesium oxide and the mixture is stirred at a predetermined temperature (for example, 303 K). It is empirically known that the CAA can serve as an index for the evaluation of magnesium oxide used as an annealing separator for grain-oriented electromagnetic steel sheets.

Patent Literature 8 discloses an invention related to the distribution of CAA at specific reaction equivalents of magnesium oxide. Specifically, the invention is directed to magnesium oxide for an annealing separator which has an activity so controlled that the CAA falls in a narrow range in each case where the final reaction degree is 20%, 40%, 60% and 70%. Further, Patent Literature 9 and Patent Literature 10 disclose inventions directed to magnesium oxide for an annealing separator wherein the activity at 40% CAA and 80% CAA and other properties such as particle size and specific surface area are limited to predetermined ranges. Furthermore, Patent Literature 11 discloses an invention directed to an annealing separator for grain-oriented electromagnetic steel sheets in which properties such as 70% CAA, 70% CAA/40% CAA ratio, particle size and specific surface area are limited to predetermined ranges. In all of these inventions, the hydratability and reactivity of magnesium oxide particles are controlled.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent Publication No. H04-025349 -   Patent Literature 2: Japanese Patent No. 2690841 -   Patent Literature 3: Japanese Patent Publication No. S54-014566 -   Patent Literature 4: Japanese Patent No. 3043975 -   Patent Literature 5: Japanese Patent Application Publication No.     H10-88244 -   Patent Literature 6: Japanese Patent No. 3021241 -   Patent Literature 7: Japanese Patent No. 3091096 -   Patent Literature 8: Japanese Patent Publication No. S57-045472 -   Patent Literature 9: Japanese Patent No. 2650817 -   Patent Literature 10: Japanese Patent No. 4192282 -   Patent Literature 11: Japanese Patent No. 3650525

SUMMARY OF INVENTION Technical Problem

The magnetic properties and insulating properties of grain-oriented electromagnetic steel sheets, as well as their market value, are affected by the performance of forsterite film, specifically, the following four points: (a) formability of forsterite film (forsterite film formation ratio), (b) film appearance, (c) film adhesion and (d) acid-removability of unreacted magnesium oxide. In other words, the properties and value of grain-oriented electromagnetic steel sheets depend on the performance of magnesium oxide for an annealing separator which is used to form a forsterite film.

Unfortunately, the conventional magnesium oxides for an annealing separator are unreliable on account of the facts that the occurrence of defective films on grain-oriented electromagnetic steel sheets cannot be perfectly avoided, and the effects cannot be achieved constantly. Thus, magnesium oxide for an annealing separator which possesses sufficient performance is not yet developed.

As described earlier, Patent Literatures 1 to 5 disclose the attempts to investigate the structures of trace element compounds contained in magnesium oxides for annealing separators. However, the use of the magnesium oxides for an annealing separator described in these literatures results in forsterite films having poor adhesion or low acid-removability of unreacted magnesium oxide.

The magnesium oxides for an annealing separator described in Patent Literatures 6 and 7 are developed focusing on the effect of halogen, in particular fluorine, in promoting the formation of forsterite film. The magnesium oxides described in these literatures show constant effects of the formation of forsterite film, but their effects are still unsatisfactory.

As discussed above, many attempts to control various physical properties of magnesium oxide for an annealing separator so as to attain constant effects of promoting the formation of forsterite film and to improve the quality of forsterite film have been made. However, there has been a demand for a further enhancement in quality so that the magnesium oxide for an annealing separator will satisfy the requirements ((a) to (d) described above) sufficiently.

The activity of magnesium oxide indicated by CAA is an indicator for the reactivity in the solid phase-liquid phase reaction between magnesium oxide and citric acid. In this solid phase-liquid phase reaction, the surface free energy is higher and the activity is enhanced with increasing number of reaction sites of the solid phase, that is, with decreasing particle size of magnesium oxide and with increasing specific surface area thereof. However, powdery particles such as magnesium oxide do not always exist as unit particles, and in many cases are aggregated and bonded to form agglutination of particles depending on production methods. In the case where the particles are collections of aggregated and agglutinated particles, the measured value of CAA does not reflect the structure of the particles as aggregates. Thus, the reactivity of an annealing separator cannot be represented accurately by CAA alone.

Further, the CAA only simulates empirically the reactivity in the solid phase-solid phase reaction between silicon dioxide and magnesium oxide which actually occurs on the surface of an electromagnetic steel sheet, based on the reactivity in the solid phase-liquid phase reaction between magnesium oxide and citric acid. Unlike a solid phase-liquid phase reaction, the forsterite formation reaction, which is a solid phase-solid phase reaction, will be significantly affected by the aggregate structure of magnesium oxide particles, typically, for example, the number of contacts between the silicon dioxide film and the magnesium oxide particles. Specifically, when the number of contacts affected by the structure of aggregated particles is small, the reaction will be insufficient even when the magnesium oxide particles have active surface. On the other hand, when a large number of contacts exist, the sufficient reaction can be occurred even when the surface of magnesium oxide particles is inactive.

As described above, the CAA used as an indicator for the characteristics of an annealing separator for an electromagnetic steel sheet is usable for the evaluation of reactivity of magnesium oxide only under limited conditions, and this indicator cannot be always said to evaluate the reactivity in a solid phase-solid phase reaction which actually occurs on the surface of an electromagnetic steel sheet. Thus, a technique which controls the solid phase-solid phase reaction in consideration of the aggregate structure of powdery particles offers a possibility that magnesium oxide, even if it shows poor activity according to the CAA evaluation, is found to have a particle aggregate structure suitable for an annealing separator. Further, such a technique which controls the solid phase-solid phase reaction in consideration of the aggregate structure of powdery particles offers a possibility that magnesium oxide which can give grain-oriented electromagnetic steel sheets with enhanced magnetic properties and insulating properties can be selected from among magnesium oxides evaluated as having good activity according to the CAA evaluation.

Therefore, it is an object of the present invention to provide magnesium oxide for an annealing separator which is useful for obtaining a grain-oriented electromagnetic steel sheet with excellent magnetic properties and insulating properties. Specifically, an object of the present invention is to provide magnesium oxide for an annealing separator which can form a forsterite film showing excellent performance in forsterite film formation ratio, film appearance, film adhesion and acid-removability of unreacted magnesium oxide on the surface of a steel sheet.

Solution to Problem

The present inventors have found that a grain-oriented electromagnetic steel sheet with excellent magnetic properties and insulating properties can be obtained by controlling the Blaine specific surface area and CAA of magnesium oxide for an annealing separator to the predetermined ranges. Then, the present inventors have completed the present invention based on the finding.

An aspect of the present invention resides in magnesium oxide for annealing separator having a Blaine specific surface area of 2.5×10³ to 7.0×10³ m²·kg⁻¹ and CAA of 50 to 170 seconds.

The use of the magnesium oxide for an annealing separator according to the present invention makes it possible to obtain a grain-oriented electromagnetic steel sheet having superior magnetic properties and insulating properties. Specifically, a forsterite film showing excellent performance in forsterite film formation ratio, film appearance, film adhesion and acid-removability of unreacted magnesium oxide can be formed on the surface of a steel sheet.

Preferably, the magnesium oxide for an annealing separator of the present invention contains 0.04 to 0.15 mass % boron and has a chlorine content of not more than 0.05 mass %. By limiting the contents of boron and chlorine to the predetermined ranges, the magnesium oxide for an annealing separator which can be used for obtaining a grain-oriented electromagnetic steel sheet with excellent magnetic properties and insulating properties can be obtained more reliably.

Another aspect of the present invention resides in an annealing separator including the above magnesium oxide for an annealing separator. By the use of the annealing separator of the present invention, a grain-oriented electromagnetic steel sheet with excellent magnetic properties and insulating properties can be manufactured.

Another aspect of the present invention resides in a method for manufacturing a grain-oriented electromagnetic steel sheet, comprising the step of forming a silicon dioxide film on a steel sheet surface, and the step of forming a forsterite film on the steel sheet surface by applying the aforementioned annealing separator onto the surface of the silicon dioxide film, and annealing the steel sheet. By the method of the present invention, a grain-oriented electromagnetic steel sheet with excellent magnetic properties and insulating properties can be manufactured.

Advantageous Effects of Invention

According to the present invention, the magnesium oxide for an annealing separator which can be used for obtaining a grain-oriented electromagnetic steel sheet with excellent magnetic properties and insulating properties can be provided. Specifically, according to the present invention, the magnesium oxide for an annealing separator can form a forsterite film showing excellent performance in forsterite film formation ratio, film appearance, film adhesion and acid-removability of unreacted magnesium oxide on the surface of a steel sheet.

DESCRIPTION OF EMBODIMENTS

Magnesium oxide for annealing separator of the present invention has a Blaine specific surface area of 2.5×10³ to 7.0×10³ m²·kg⁻¹ and CAA of 50 to 170 seconds. Here, the Blaine specific surface area is the specific surface area measured by the Blaine method described in JIS R5201: 2015, “8.1 Specific surface area test”. The CAA (citric acid activity) means the time required for a 0.4 N aqueous citric acid solution to reach the end of reaction when it is mixed with 40% final reaction equivalent of magnesium oxide and the mixture is stirred at a temperature of 303 K, that is, the time required for consuming the citric acid and neutralizing the solution.

In the Blaine method, the specific surface area is measured by passing air through a layer of packed particles. Because of this fact, the method cannot measure the surface area of fine pores which are not accessible for the flow of air. Thus, the Blaine method can measure the specific surface area (the Blaine specific surface area) of aggregated particles alone without the surface area of primary particles forming the aggregated particles.

The CAA empirically simulates the reactivity in the solid phase-solid phase reaction between silicon dioxide and magnesium oxide which actually occurs on the surface of an electromagnetic steel sheet, based on the solid phase-liquid phase reaction. The reactivity of magnesium oxide particles including primary particles can be thus measured.

When the Blaine specific surface area of the magnesium oxide is less than 2.5×10³ m²·kg⁻¹, the aggregated particles of the magnesium oxide are so coarse and have less contacts with the steel sheet, and become less reactive, causing a decrease in (a) forsterite film formation ratio. Further, because the forsterite film containing coarse particles of aggregated magnesium oxide is formed, the thickness is nonuniform. Consequently, (b) film appearance and/or (c) adhesion of the forsterite film on a grain-oriented electromagnetic steel sheet is deteriorated.

When the Blaine specific surface area of the magnesium oxide is larger than 7.0×10³ m²·kg⁻¹, the aggregated particles of the magnesium oxide are so small in particle size and have more contacts with the steel sheet and become too reactive, failing to form a uniform forsterite film. Consequently, (b) film appearance and/or (c) adhesion of the forsterite film on a grain-oriented electromagnetic steel sheet is deteriorated.

When the CAA of the magnesium oxide is more than 170 seconds, the primary particle size of magnesium oxide is so large that the magnesium oxide particles exhibit low reactivity, causing a decrease in (a) forsterite film formation ratio. Further, residues will remain after removal with acids because of the particles being coarse. That is, (d) acid-removability is low.

When the CAA of the magnesium oxide is less than 50 seconds, the primary particle size of magnesium oxide is so small that the magnesium oxide particles show too high reactivity and fail to form a uniform forsterite film. Consequently, (b) film appearance and/or (c) adhesion of the forsterite film is deteriorated.

As described above, by controlling the Blaine specific surface area and CAA of aggregated particles of the magnesium oxide, the present invention is enabled to achieve with high reliability an outstanding forsterite film-forming ability that conventional magnesium oxides for an annealing separator have not succeeded. Here, the outstanding forsterite film-forming ability is indicated by the levels of forsterite film formation ratio, film adhesion and acid-removability of unreacted magnesium oxide and film appearance in the manufacturing of a grain-oriented electromagnetic steel sheet.

By using the magnesium oxide for an annealing separator of the present invention, grain-oriented electromagnetic steel sheet having excellent insulating properties and magnetic properties can be manufactured.

In the present invention, the magnesium oxide may be produced by a known method. For example, the magnesium oxide may be produced in such a manner that slurry of calcium hydroxide is added to and reacted with an aqueous solution of magnesium chloride as a raw material to form magnesium hydroxide, and the magnesium hydroxide is filtrated, washed with water, dried, and calcined in a heating furnace to give magnesium oxide, followed by crushing the resultant magnesium oxide to a desired particle size.

The calcium hydroxide may be replaced by an alkaline compound having hydroxyl such as sodium hydroxide and potassium hydroxide. Alternatively, the magnesium oxide may be produced by the Aman process in which an magnesium chloride-containing aqueous solution such as seawater, brine and bittern is introduced into a reactor, and magnesium oxide and hydrochloric acid are formed directly at 1773 to 2273 K, followed by crushing the resultant magnesium oxide to desired particle size.

Still alternatively, the magnesium oxide may be produced by hydrating magnesium oxide resulting from the calcination of mineral magnesite, and calcining the resultant magnesium hydroxide followed by crushing the resultant magnesium oxide to desired particle size.

In the present invention, the Blaine specific surface area and the CAA of the magnesium oxide may be adjusted as follows. The Blaine specific surface area and the CAA of the magnesium oxide may be adjusted by adjusting the reaction temperature and the concentration of an alkali source in a manufacturing process of magnesium hydroxide, and thereby controlling the primary particle size and secondary particle size of the magnesium hydroxide. Alternatively, the Blaine specific surface area and the CAA of the magnesium oxide may be adjusted by controlling the temperature and time of the calcination of magnesium hydroxide having a controlled particle size. Still alternatively, the Blaine specific surface area and CAA may be adjusted by measuring the Blaine specific surface area and the CAA after a crushing operation, and performing calcination several times. Further, the Blaine specific surface area and the CAA of the magnesium oxide may be adjusted by crushing calcined magnesium oxide with use of a crushing machine such as a jaw crusher, a gyratory crusher, a cone crusher, an impact crusher, a roll crusher, a cutter mill, a stamp mill, a ring mill, a roller mill, a jet mill, a hammer mill, a rotary mill, a vibration mill, a planetary mill and a ball mill.

The Blaine specific surface area and the CAA of the magnesium oxide may be adjusted by classifying the magnesium oxide particles with a classifier so that the desired Blaine specific surface area and the CAA will be obtained.

The optimum conditions of the crushing machine for obtaining the ranges of the Blaine specific surface area and the CAA of the magnesium oxide of the present invention are variable depending on the type and performance (power) of the crushing machine. When the crushing intensity increases, the Blaine specific surface area increases, and the CAA reactivity is enhanced. In contrast, when the crushing intensity decreases, the Blaine specific surface area decreases, and the CAA reactivity is lowered. The use of a classifier is not always necessary, but the combined use allows these properties to be controlled within a wider range.

In the case where the magnesium oxide obtained by the aforementioned method is measured and turns out to have an excessively large Blaine specific surface area and excessively high CAA, the properties may be adjusted by combining and mixing the magnesium oxide with other magnesium oxide having a smaller Blaine specific surface area and smaller CAA.

The magnesium oxide for an annealing separator of the present invention preferably has a BET specific surface area of 12.0×10³ to 25.0×10³ m²·kg⁻¹, more preferably 12.0×10³ to 23.0×10³ m²·kg⁻¹, and still more preferably 14.0×10³ to 20.0×10³ m²·kg⁻¹. Here, the BET specific surface area is the specific surface area measured by the nitrogen gas adsorption method (the BET method). When the BET specific surface area is in the above range, the magnesium oxide has an appropriate primary particle size.

In the present invention, the contents of trace components may be controlled during the steps of production of a crude product. In such a case, trace components may be added in wet manner. Specifically, for example, using an aqueous magnesium chloride solution having the contents of trace components analyzed in advance as a raw material, during the step of forming magnesium hydroxide by adding an alkaline aqueous solution or slurry having hydroxyl into the aqueous magnesium chloride solution and reacting them with each other, the contents of trace components may be adjusted to predetermined ranges. For example, when calcium (Ca) is added, calcium in the form of oxide, hydroxide, carbonate salt, nitrate salt, sulfate salt, silicate salt and phosphate salt may be used. When phosphorus (P) is added, phosphoric acid, metaphosphoric acid, phosphonic acid and phosphorous acid, and alkali metal salts, alkaline earth metal salts and ammonium salts thereof may be used. When boron (B) is added, boric acid, alkali metal borate salts, ammonium borate salts, alkali metal metaborate salts, boron dioxide and the like may be used. When sulfur (S) is added, sulfuric acid and sulfurous acid, and alkali metal salts, alkaline earth metal salts and ammonium salts thereof may be used. When fluorine (F) is added, magnesium fluoride and the like may be used. When chlorine (Cl) is added, magnesium chloride and the like may be used. When silicon (Si) is added, alkali metal silicate salts, alkaline earth metal silicate salts and colloidal silica may be used.

In the present invention, the contents of trace components may be controlled during the steps of production of a crude product. In such a case, trace components may be added in wet manner. Specifically, for example, using an aqueous magnesium chloride solution having the contents of trace components analyzed in advance as a raw material, during the step of forming magnesium hydroxide by adding an alkaline aqueous solution or slurry having hydroxyl into the aqueous magnesium chloride solution and reacting them with each other, the contents of trace components may be adjusted to predetermined ranges. For example, when calcium (Ca) is added, calcium in the form of oxide, hydroxide, carbonate salt, nitrate salt, sulfate salt, silicate salt and phosphate salt may be used. When phosphorus (P) is added, phosphoric acid, metaphosphoric acid, phosphonic acid and phosphorous acid, and alkali metal salts, alkaline earth metal salts and ammonium salts thereof may be used. When boron (B) is added, boric acid, alkali metal borate salts, ammonium borate salts, alkali metal metaborate salts, boron dioxide and the like may be used. When sulfur (S) is added, sulfuric acid and sulfurous acid, and alkali metal salts, alkaline earth metal salts and ammonium salts thereof may be used. When fluorine (F) is added, magnesium fluoride and the like may be used. When chlorine (Cl) is added, magnesium chloride and the like may be used. When silicon (Si) is added, alkali metal silicate salts, alkaline earth metal silicate salts and colloidal silica may be used.

The magnesium oxide of the present invention may contain trace components such as calcium (Ca), phosphorus (P), boron (B), sulfur (S), fluorine (F) and chlorine (Cl). When the magnesium oxide of the present invention contains calcium (Ca), the content of calcium is preferably 0.2 to 2.0 mass % in terms of CaO. When the magnesium oxide of the present invention contains phosphorus (P), the content of phosphorus is preferably 0.03 to 0.15 mass % in terms of P₂O₃. When the magnesium oxide of the present invention contains boron (B), the content of boron is preferably 0.04 to 0.15 mass %. When the magnesium oxide of the present invention contains sulfur (S), the content of sulfur is preferably 0.01 to 1.5 mass % in terms of SO₃. When the magnesium oxide of the present invention contains fluorine (F), the content of fluorine is preferably not more than 0.05 mass %. When the magnesium oxide of the present invention contains chlorine (Cl), the content of chlorine is preferably not more than 0.05 mass %. When the magnesium oxide of the present invention contains silicon (Si), the content of silicon is preferably 0.05 to 0.5 mass %. Here, “mass %” in the present specification means the same as “wt %”.

The contents of trace components in the present invention may be also controlled by removing such components during the steps of production of a crude product. In such a case, the trace components may be removed by adding an acid during the aforementioned step of forming magnesium hydroxide, or by performing filtration and washing the product with water repeatedly after the aforementioned step of forming magnesium hydroxide.

Further, the contents of trace components may be controlled before the final calcination with respect to the product obtained such as magnesium hydroxide. In this case, they may be controlled by analyzing the magnesium hydroxide obtained to determine the contents of trace elements and supplementing any trace components or, if any trace elements are present in excessively large amounts, combining and mixing the magnesium hydroxide with other magnesium hydroxide having lower contents of such trace elements, followed by calcination.

The grain-oriented electromagnetic steel sheet of the present invention may be manufactured by the following method. A steel sheet for the grain-oriented electromagnetic steel sheet may be manufactured by hot-rolling a silicon steel slab containing 2.5 to 4.5% silicon (Si), pickling the steel sheet, and adjusting the thickness of the steel sheet to a predetermined one by performing heavy cold-rolling or two passes of cold-rolling with intermediate annealing between the passes. Next, the coil of the cold-rolled steel sheet is subjected to recrystallization-annealing, which also serves as decarburization in a wet hydrogen atmosphere at 923 to 1173 K. During this process, an oxide film based on silicon dioxide is formed on the surface of the steel sheet. Next, an annealing separator including the magnesium oxide for an annealing separator of the present invention is uniformly dispersed in water to give slurry. This slurry is continuously applied onto the surface of the oxide-coated steel sheet by roll coating or spray coating, and is dried at about 573 K. The thus-treated steel sheet is finish-annealed, for example, at 1473 K for 20 hours to form a forsterite film (a Mg₂SiO₄ film) on the steel sheet surface. The forsterite film is an insulating film, and also imparts tension to the surface of the steel sheet, and reduces the iron loss of the grain-oriented electromagnetic steel sheet to enhance magnetic properties.

EXAMPLES

The present invention will be described in detail based on Examples below. However, it should be construed that these Examples do not limit the scope of the invention in any way.

<Testing Methods>

(1) Method for Measuring Blaine Specific Surface Area

The Blaine specific surface area was measured in accordance with JIS R5201: 2015 (8. Fineness test, 8.1 Specific surface area test) using a Blaine air permeability apparatus (C-202B manufactured by Nishinihon Shikenki). The measurement was performed at room temperature 298±1 K, and the porosity was set at 0.80.

(2) Method for Measuring Boron (B) Content

A measurement sample was added to 12 N hydrochloric acid (special grade chemical) and was completely dissolved by heating. The boron (B) content was then measured using an ICP optical emission spectrometer (PS3520 VDD manufactured by Hitachi High-Tech Science Corporation).

(3) Method for Measuring Chlorine (Cl) Content

The content of chlorine (Cl) was measured by dissolving a sample into an acid and determining the mass with use of a spectrophotometer (UV-2550 manufactured by Shimadzu Corporation), thereby calculating the concentration in the sample.

(4) CAA Measurement Method

1×10⁻⁴ m³ 0.4 N citric acid solution and an appropriate amount (2×10⁻⁶ m³) of 1% phenolphthalein solution as an indicator were added to a 2×10⁻⁴ m³ beaker. 40% Final reaction equivalent of magnesium oxide was added to the citric acid solution and the mixture was stirred with a magnetic stirrer at 700 rpm while controlling the liquid temperature at 303 K. The time required for the reaction to complete, that is, the time required for consuming the citric acid and neutralizing the solution was measured.

(5) Forsterite Film Formation Ratio

In light of the mechanism of forsterite formation represented by the reaction formula: 2MgO+SiO₂→Mg₂SiO₄, a mixture containing magnesium oxide powder and amorphous silicon dioxide in a molar ratio of 2:1 was prepared. The mixture weighing 0.8×10⁻³ kg was molded under a pressure of 50 MPa, then a molded body having a diameter of 15×10⁻³ m and a height of about 3×10⁻³ m was obtained. Next, this compact was calcined in a nitrogen atmosphere at 1473 K for 4.0 hours. The amount of forsterite formed in the sintered body was quantitatively analyzed by X-ray diffractometry. It is likely that 90% or higher formation ratio indicates that the magnesium oxide has sufficient reactivity and will form a quality forsterite film.

(6) Appearance of Forsterite Film

Samples for the testing of forsterite film appearance, forsterite film adhesion and acid-removability of unreacted magnesium oxide were supplied from a steel sheet which had been produced by hot-rolling and cold-rolling a silicon steel slab for a grain-oriented electromagnetic steel sheet in a known manner to a final thickness of 0.28×10⁻³ m and thereafter subjecting the steel sheet to decarburization-annealing in a wet atmosphere containing 25% nitrogen plus 75% hydrogen. The chemical composition of the steel sheet before the decarburization annealing included, in mass %, C: 0.01%, Si: 3.29%, Mn: 0.09%, Al: 0.03%, S: 0.07% and N: 0.0053%, the balance being Fe and inevitable impurities. Magnesium oxide was applied onto the electromagnetic steel sheet, and film characteristics of the forsterite film were examined. Specifically, the slurry of the magnesium oxide of the present invention or magnesium oxide of Comparative Example was applied to the steel sheet so that the dry mass would be 14.0×10⁻³ kg·m⁻². After drying, final finish-annealing was performed at 1473 K for 20 hours. After the completion of the final finish annealing, the steel sheet was cooled, washed with water, pickled with an aqueous hydrochloric acid solution, washed with water again, and dried. The appearance of the film was evaluated with respect to the film after the washing. Specifically, the appearance was evaluated as follows: ⊚ when a gray uniform forsterite film had been formed with a large thickness; ◯ when the film was uniform but was slightly thin; Δ when the film was nonuniform and thin but covered the underlying steel sheet without exposure; and × when the film was nonuniform and very thin, and had portions where the underlying steel sheet were visibly exposed.

(7) Adhesion of Forsterite Film

The adhesion of forsterite film was evaluated based on the condition of the film before the washing. Specifically, the adhesion was evaluated as follows: ⊚ when the film was uniform and was free from separation; ◯ when the film was slightly nonuniform but was free from separation; Δ when the film was nonuniform and had been exfoliated to leave pinholes; and × when the film was nonuniform and had been visibly exfoliated.

(8) Acid-Removability of Unreacted Magnesium Oxide

The acid-removability of unreacted magnesium oxide (also simply written as “acid-removability”) was evaluated based on the condition of the film after the washing. Specifically, acid-removability was evaluated as follows: ⊚ when the unreacted magnesium oxide had been perfectly removed; ◯ when the film had different color densities arising from slight residues of unreacted magnesium oxide, although no visible residues of unreacted magnesium oxide were present; Δ when there were visible dots of unreacted magnesium oxide; and × when the unreacted magnesium oxide visibly remained.

SYNTHETIC EXAMPLES USING REAGENTS Synthetic Example 1

Magnesium chloride (special grade chemical) was dissolved into pure water to give a 0.5×10³ mol·m⁻³ aqueous magnesium chloride solution. Next, calcium hydroxide (special grade chemical) was added to pure water to give a 0.5×10³ mol·m⁻³ calcium hydroxide dispersion. The aqueous magnesium chloride solution and the calcium hydroxide dispersion were mixed together to give 1.0×10⁻³ m³ of a mixture liquid containing MgCl₂/Ca(OH)₂ in a molar ratio of 1.1. Thereafter, an aqueous boric acid solution adjusted to 0.3×10³ mol·m⁻³ with pure water was added to the mixture liquid so that the boron (B) content in the final magnesium oxide would be 0.06 mass %. The reaction was performed at 363 K for 5.5 hours while stirring the mixture at 600 rpm with a four-blade stirrer, thus giving magnesium hydroxide slurry. Thereafter, the magnesium hydroxide slurry was filtered. The residue was washed with a 100-fold mass of pure water relative to the mass of the expected magnesium hydroxide, and was dried at 378 K for 12 hours to afford magnesium hydroxide powder. Further, the magnesium hydroxide powder obtained was calcined with an electric furnace at 1373 K for 2.5 hours. Consequently, magnesium oxide having a Blaine specific surface area of 1.4×10³ m²·kg⁻¹ and a CAA of 172 seconds was obtained.

Synthetic Example 2

Magnesium chloride (special grade chemical) was dissolved into pure water to give a 0.5×10³ mol·m⁻³ aqueous magnesium chloride solution. Next, calcium hydroxide (special grade chemical) was added to pure water to give a 0.5×10³ mol·m⁻³ calcium hydroxide dispersion. The aqueous magnesium chloride solution and the calcium hydroxide dispersion were mixed together to give 1.0×10⁻³ m³ of a mixture liquid containing MgCl₂/Ca(OH)₂ in a molar ratio of 1.1. Thereafter, an aqueous boric acid solution adjusted to 0.3×10³ mol·m⁻³ with pure water was added to the mixture liquid so that the boron content in the final magnesium oxide would be 0.06 mass %. The reaction was performed at 363 K for 6.0 hours while stirring the mixture at 300 rpm with a four-blade stirrer, thus giving magnesium hydroxide slurry. Thereafter, the magnesium hydroxide slurry was filtered. The residue was washed with a 40-fold mass of pure water relative to the mass of the expected magnesium hydroxide, and was dried at 378 K for 12 hours to afford magnesium hydroxide powder. The magnesium hydroxide powder obtained was calcined with an electric furnace at 1173 K for 1 hour. Consequently, magnesium oxide having a Blaine specific surface area of 2.0×10³ m²·kg⁻¹ and a CAA of 89 seconds was obtained.

Synthetic Example 3

Magnesium chloride (special grade chemical) was dissolved into pure water to give a 0.5×10³ mol·m⁻³ aqueous magnesium chloride solution. Next, calcium hydroxide (special grade chemical) was added to pure water to give a 0.5×10³ mol·m⁻³ calcium hydroxide dispersion. The aqueous magnesium chloride solution and the calcium hydroxide dispersion were mixed together to give 1.0×10⁻³ m³ of a mixture liquid containing MgCl₂/Ca(OH)₂ in a molar ratio of 1.1. Thereafter, an aqueous boric acid solution adjusted to 0.3×10³ mol·m⁻³ with pure water was added to the mixture liquid so that the boron content in the final magnesium oxide would be 0.06 mass %. The reaction was performed at 363 K for 3.0 hours while stirring the mixture at 600 rpm with a four-blade stirrer, thus giving magnesium hydroxide slurry. Thereafter, the magnesium hydroxide slurry was filtered. The residue was washed with a 40-fold mass of pure water relative to the mass of the expected magnesium hydroxide, and was dried at 378 K for 12 hours to afford magnesium hydroxide powder. The magnesium hydroxide powder obtained was calcined with an electric furnace at 1073 K for 1.5 hours. Consequently, magnesium oxide having a Blaine specific surface area of 3.2×10³ m²·kg⁻¹ and a CAA of 45 seconds was obtained.

Synthetic Example 4

Magnesium chloride (special grade chemical) was dissolved into pure water to give a 0.5×10³ mol·m⁻³ aqueous magnesium chloride solution. Next, calcium hydroxide (special grade chemical) was added to pure water to give a 0.5×10³ mol·m⁻³ calcium hydroxide dispersion. The aqueous magnesium chloride solution and the calcium hydroxide dispersion were mixed together to give 1.0×10⁻³ m³ of a mixture liquid containing MgCl₂/Ca(OH)₂ in a molar ratio of 1.1. Thereafter, an aqueous boric acid solution adjusted to 0.3×10³ mol·m⁻³ with pure water was added to the mixture liquid so that the boron content in the final magnesium oxide would be 0.06 mass %. The reaction was performed at 343 K for 5.0 hours while stirring the mixture at 300 rpm with a four-blade stirrer, thus giving magnesium hydroxide slurry. Thereafter, the magnesium hydroxide slurry was filtered. The residue was washed with a 100-fold mass of pure water relative to the mass of the expected magnesium hydroxide, and was dried at 378 K for 12 hours to afford magnesium hydroxide powder. The magnesium hydroxide powder obtained was calcined with an electric furnace at 1123 K for 1.5 hours. Consequently, magnesium oxide having a Blaine specific surface area of 7.6×10³ m²·kg⁻¹ and a CAA of 51 seconds was obtained.

Synthetic Example 5

Magnesium chloride (special grade chemical) was dissolved into pure water to give a 0.5×10³ mol·m⁻³ aqueous magnesium chloride solution. Next, calcium hydroxide (special grade chemical) was added to pure water to give a 0.5×10³ mol·m⁻³ calcium hydroxide dispersion. The aqueous magnesium chloride solution and the calcium hydroxide dispersion were mixed together to give 1.0×10⁻³ m³ of a mixture liquid containing MgCl₂/Ca(OH)₂ in a molar ratio of 1.1. Thereafter, an aqueous boric acid solution adjusted to 0.3×10³ mol·m⁻³ with pure water was added to the mixture liquid so that the boron content in the final magnesium oxide would be 0.06 mass %. The reaction was performed at 363 K for 6.0 hours while stirring the mixture at 300 rpm with a four-blade stirrer, thus giving magnesium hydroxide slurry. Thereafter, the magnesium hydroxide slurry was filtered. The residue was washed with a 100-fold mass of pure water relative to the mass of the expected magnesium hydroxide, and was dried at 378 K for 12 hours to afford magnesium hydroxide powder. The magnesium hydroxide powder obtained was calcined with an electric furnace at 1173 K for 1.5 hours. Consequently, magnesium oxide having a Blaine specific surface area of 7.5×10³ m²·kg⁻¹ and a CAA of 85 seconds was obtained.

Table 1 shows the components of the magnesium oxides of Synthetic Examples 1 to 5 produced from the reagents.

TABLE 1 Blaine specific Chemical composition surface area CAA B Cl (10³ m² · kg⁻¹) (sec) (mass %) (mass %) Syn. Ex. 1 1.4 172 0.054 0.029 Syn. Ex. 2 2.0 89 0.065 0.030 Syn. Ex. 3 3.2 45 0.060 0.025 Syn. Ex. 4 7.6 51 0.055 0.023 Syn. Ex. 5 7.5 85 0.056 0.024

EXAMPLES AND COMPARATIVE EXAMPLES USING REAGENTS Examples 1 to 5 and Comparative Examples 1 to 5

The magnesium oxides of Synthetic Examples 1 to 5 were mixed in accordance with the formulations shown in Table 2 to afford magnesium oxides of Examples 1 to 5 and Comparative Examples 1 to 5.

The magnesium oxides thus obtained were each applied to the decarburized and annealed steel sheet, and finish-annealing was performed to form a forsterite film on the surface of the steel sheet. The steel sheets thus obtained were tested to evaluate the forsterite film formation ratio, the film appearance, the film adhesion and the acid-removability of unreacted magnesium oxide. The results are shown in Table 2.

TABLE 2 Blaine Forsterite Chemical specific formation composition surface area CAA ratio Film Acid- B Cl Compounding ratio data (10³ m² · kg⁻¹) (sec) (%) appearance Adhesion removability (mass %) (mass %) Ex. 1 Syn. Ex. 2 50 mass % 4.4 92 90.4 ⊚ ◯ ◯ 0.061 0.022 Syn. Ex. 5 50 mass % Ex. 2 Syn. Ex. 2 80 mass % 3.2 76 90.6 ⊚ ⊚ ⊚ 0.067 0.023 Syn. Ex. 4 20 mass % Ex. 3 Syn. Ex. 2 30 mass % 5.8 64 91.2 ⊚ ◯ ⊚ 0.060 0.034 Syn. Ex. 4 70 mass % Ex. 4 Syn. Ex. 1 20 mass % 5.7 105 90.3 ⊚ ⊚ ⊚ 0.064 0.036 Syn. Ex. 4 80 mass % Ex. 5 Syn. Ex. 1 45 mass % 3.1 126 90.8 ⊚ ◯ ◯ 0.059 0.031 Syn. Ex. 4 55 mass % Comp. Syn. Ex. 1 100 mass % 1.4 172 79.9 Δ Δ X 0.054 0.029 Ex. 1 Comp. Syn. Ex. 2 100 mass % 2 89 88.4 Δ X ◯ 0.065 0.030 Ex. 2 Comp. Syn. Ex. 3 100 mass % 3.2 45 90.1 X Δ Δ 0.060 0.025 Ex. 3 Comp. Syn. Ex. 4 100 mass % 7.6 51 88.1 X X Δ 0.055 0.023 Ex. 4 Comp. Syn. Ex. 5 100 mass % 7.5 85 86.4 Δ Δ ∘ 0.056 0.024 Ex. 5

As clear from Table 2, the forsterite films formed using the magnesium oxide (Examples 1 to 5) which had a Blaine specific surface area and CAA within the predetermined ranges were excellent in all aspects of forsterite film formation ratio, film appearance, film adhesion and acid-removability of unreacted magnesium oxide. Thus, it can be said that these forsterite films are uniform and sufficiently thick films.

In contrast, the forsterite films formed using the magnesium oxide (Comparative Examples 1 to 5) which had been obtained without adjusting the Blaine specific surface area and the CAA of magnesium oxide and consequently had a Blaine specific surface area and CAA outside the predetermined ranges failed to satisfy any of the characteristics of forsterite film formation ratio, film appearance, film adhesion and acid-removability of unreacted magnesium oxide. The desired steel sheets were not obtained as a result.

EXAMPLES AND COMPARATIVE EXAMPLES USING OTHER THAN REAGENTS Example 6

An aqueous boric acid solution adjusted to 0.3×10³ mol·m⁻³ with pure water was added to bittern so that the boron content in the final magnesium oxide would be 0.07 mass %. This bittern contained 2.0×10³ mol·m⁻³ magnesium ions. Calcium hydroxide slurry was added to the bittern so that the magnesium hydroxide concentration after the reaction would be 2.0×10³ mol·m⁻³. The reaction was performed at 323 K for 7.0 hours while stirring the mixture at 600 rpm. Thereafter, the mixture was filtered through a filter press. The residue was washed with water and dried to afford magnesium hydroxide. The magnesium hydroxide was calcined with a rotary kiln at 1373 K for 0.5 hours to give magnesium oxide. The magnesium oxide obtained was crushed with an impact crusher (a jet mill) to adjust the Blaine specific surface area and the BET specific surface area to the predetermined ranges. The final magnesium oxide had a Blaine specific surface area of 3.8×10³ m²·kg⁻¹ and CAA of 79 seconds.

Example 7

An aqueous boric acid solution adjusted to 0.3×10³ mol·m⁻³ with pure water was added to bittern so that the boron content in the final magnesium oxide would be 0.07 mass %. This bittern contained 2.0×10³ mol·m⁻³ magnesium ions. Calcium hydroxide slurry was added to the bittern so that the magnesium hydroxide concentration after the reaction would be 2.0×10³ mol·m⁻³, and stirred at 600 rpm. The reaction was performed at 353 K for 2.0 hours. Thereafter, the mixture was filtered through a filter press. The residue was washed with water and dried to afford magnesium hydroxide. The magnesium hydroxide was calcined with a rotary kiln at 1373 K for 0.5 hours to give magnesium oxide. The magnesium oxide obtained was crushed with an impact crusher (a jet mill) to adjust the Blaine specific surface area and the BET specific surface area to the predetermined ranges. The final magnesium oxide had a Blaine specific surface area of 4.1×10³ m²·kg⁻¹ and CAA of 73 seconds.

Example 8

An aqueous boric acid solution adjusted to 0.3×10³ mol·m⁻³ with pure water was added to bittern so that the boron content in the final magnesium oxide would be 0.06 mass %. This bittern contained 2.0×10³ mol·m⁻³ magnesium ions. Calcium hydroxide slurry was added to the bittern so that the magnesium hydroxide concentration after the reaction would be 1.0×10³ mol·m⁻³. The reaction was performed at 333 K for 20.0 hours while stirring the mixture at 600 rpm. Thereafter, the mixture was filtered through a filter press. The residue was washed with water and dried to afford magnesium hydroxide. The magnesium hydroxide was calcined with a rotary kiln at 1373 K for 0.5 hours to give magnesium oxide. The magnesium oxide obtained was crushed with an impact crusher (a jet mill) to adjust the Blaine specific surface area and the BET specific surface area to the predetermined ranges. The final magnesium oxide had a Blaine specific surface area of 5.3×10³ m²·kg⁻¹ and CAA of 77 seconds.

Example 9

Calcium hydroxide was added to seawater so that the magnesium hydroxide concentration after the reaction would be 0.05×10³ mol·m⁻³. An aqueous boric acid solution adjusted to 0.3×10³ mol·m⁻³ with pure water was added to the seawater so that the boron content in the final magnesium oxide would be 0.06 mass %. The reaction was performed at 323 K for 20.0 hours to form magnesium hydroxide. 5.0 hours before the end of the reaction, 0.02 mass % polymer coagulant was added. Thereafter, the magnesium hydroxide was recovered by filtration through a filter press, washed with water and dried. Next, the magnesium hydroxide was calcined with a rotary kiln at 1373 K for 1.0 hour to give magnesium oxide. The magnesium oxide obtained was crushed with an impact crusher (a jet mill) to adjust the Blaine specific surface area and the BET specific surface area to the predetermined ranges. The final magnesium oxide had a Blaine specific surface area of 3.1×10³ m²·kg⁻¹ and CAA of 90 seconds.

Example 10

Calcium hydroxide was added to seawater so that the magnesium hydroxide concentration after the reaction would be 0.05×10³ mol·m⁻³. An aqueous boric acid solution adjusted to 0.3×10³ mol·m⁻³ with pure water was added to the seawater so that the boron content in the final magnesium oxide would be 0.06 mass %. The reaction was performed at 343 K for 20.0 hours to form magnesium hydroxide. 5 hours before the end of the reaction, 0.02 mass % polymer coagulant was added. Thereafter, the magnesium hydroxide was recovered by filtration through a filter press, washed with water and dried. Next, the magnesium hydroxide was calcined with a rotary kiln at 1373 K for 1 hour to give magnesium oxide. The magnesium oxide obtained was crushed with an impact crusher (a jet mill) to adjust the Blaine specific surface area and the BET specific surface area to the predetermined ranges. The final magnesium oxide had a Blaine specific surface area of 4.7×10³ m²·kg⁻¹ and a CAA of 93 seconds.

Comparative Example 6

An aqueous boric acid solution adjusted to 0.3×10³ mol·m⁻³ with pure water was added to bittern so that the boron content in the final magnesium oxide would be 0.06 mass %. This bittern contained 2.0×10³ mol·m⁻³ magnesium ions. Calcium hydroxide slurry was added to the bittern so that the magnesium hydroxide concentration after the reaction would be 0.8×10³ mol·m⁻³, and stirred at 600 rpm. The reaction was performed at 333 K for 10.0 hours. Thereafter, the mixture was filtered through a filter press. The residue was washed with water and dried to afford magnesium hydroxide. The magnesium hydroxide was calcined with a rotary kiln at 1473 K for 1.0 hour to give magnesium oxide. The magnesium oxide obtained was crushed with an impact crusher (a jet mill) to adjust the Blaine specific surface area and the BET specific surface area to the predetermined ranges. The final magnesium oxide had a Blaine specific surface area of 3.3×10³ m²·kg⁻¹ and a CAA of 183 seconds.

Comparative Example 7

An aqueous boric acid solution adjusted to 0.3×10³ mol·m⁻³ with pure water was added to bittern so that the boron content in the final magnesium oxide would be 0.06 mass %. This bittern contained 2.0×10³ mol·m⁻³ magnesium ions. Calcium hydroxide slurry was added to the bittern so that the magnesium hydroxide concentration after the reaction would be 0.8×10³ mol·m⁻³, and stirred at 600 rpm. The reaction was performed at 353 K for 6.0 hours. Thereafter, the mixture was filtered through a filter press. The residue was washed with water and dried to afford magnesium hydroxide. The magnesium hydroxide was calcined with a rotary kiln at 1473 K for 0.5 hours to give magnesium oxide. The magnesium oxide obtained was crushed with an impact crusher (a jet mill) to adjust the Blaine specific surface area and the BET specific surface area to the predetermined ranges. The final magnesium oxide had a Blaine specific surface area of 2.0×10³ m²·kg⁻¹ and a CAA of 133 seconds.

Comparative Example 8

An aqueous boric acid solution adjusted to 0.3×10³ mol·m⁻³ with pure water was added to bittern so that the boron content in the final magnesium oxide would be 0.06 mass %. This bittern contained 2.0×10³ mol·m⁻³ magnesium ions. Calcium hydroxide slurry was added to the bittern so that the magnesium hydroxide concentration after the reaction would be 0.8×10³ mol·m⁻³, and stirred at 600 rpm. The reaction was performed at 343 K for 8.0 hours. Thereafter, the mixture was filtered through a filter press. The residue was washed with water and dried to afford magnesium hydroxide. The magnesium hydroxide was calcined with a rotary kiln at 1473 K for 2.0 hours to give magnesium oxide. The magnesium oxide obtained was crushed with an impact crusher (a jet mill) to adjust the Blaine specific surface area and the BET specific surface area to the predetermined ranges. The final magnesium oxide had a Blaine specific surface area of 1.2×10³ m²·kg⁻¹ and a CAA of 214 seconds.

Table 3 shows the magnesium oxides of Examples 6 to 10 and Comparative Examples 6 to 8 obtained as described above.

The magnesium oxides obtained were each applied to the decarburized and annealed steel sheet, and annealing was performed to form a forsterite film on the surface of the steel sheet. The steel sheets thus obtained were tested to evaluate the forsterite film formation ratio, the film appearance, the film adhesion and the acid-removability of unreacted magnesium oxide. The results are described shown in Table 3.

TABLE 3 Forsterite Chemical Blaine specific formation composition surface area CAA ratio Film Acid- B Cl (10³ m2 · kg⁻¹) (sec) (%) appearance Adhesion removability (mass %) (mass %) Ex. 6 3.8 79 92.5 ⊚ ⊚ ⊚ 0.078 0.020 Ex. 7 4.1 73 91.9 ⊚ ◯ ◯ 0.080 0.020 Ex. 8 5.3 77 90.2 ⊚ ⊚ ◯ 0.060 0.027 Ex. 9 3.1 90 91.2 ◯ ⊚ ⊚ 0.067 0.021 Ex. 10 4.7 93 94.3 ⊚ ⊚ ◯ 0.061 0.022 Comp. 3.3 183 92.2 ◯ ◯ X 0.062 0.027 Ex. 6 Comp. 2.0 133 87.9 X Δ Δ 0.061 0.025 Ex. 7 Comp. 1.2 214 85.4 X X Δ 0.060 0.026 Ex. 8

As clear from Table 3, the forsterite films formed using the magnesium oxides (Examples 6 to 10) which had been synthesized from seawater and bittern, industrial raw materials, and had a Blaine specific surface area and CAA in the predetermined ranges were shown to be excellent in forsterite film formation ratio (90% or higher). Further, these forsterite films were clearly shown to be excellent in all other aspects of film appearance, film adhesion and acid-removability of unreacted magnesium oxide.

In contrast, the forsterite films formed using the magnesium oxide (Comparative Examples 6 to 8) which had been obtained without adjusting the Blaine specific surface area and the CAA of magnesium oxide and consequently had a Blaine specific surface area and CAA outside the predetermined ranges failed to satisfy any of the characteristics of forsterite film formation ratio, film appearance, film adhesion and acid-removability of unreacted magnesium oxide. The desired steel sheets were not obtained as a result.

From the above, it has been demonstrated that the magnesium oxides for a annealing separator of the present invention allow for the manufacturing of a grain-oriented electromagnetic steel sheets having excellent insulating properties and magnetic properties. 

The invention claimed is:
 1. Magnesium oxide for an annealing separator having a Blaine specific surface area of 2.5×10³ to 7.0×10³ m²·kg⁻¹ and citric acid activity (CAA) of 50 to 170 seconds.
 2. The magnesium oxide for an annealing separator according to claim 1, wherein the magnesium oxide contains 0.04 to 0.15 mass % boron and has a chlorine content of not more than 0.05 mass %.
 3. An annealing separator comprising the magnesium oxide for an annealing separator according to claim
 1. 4. A method for manufacturing a grain-oriented electromagnetic steel sheet, comprising: the step of forming a silicon dioxide film on a steel sheet surface; and the step of forming a forsterite film on the steel sheet surface by applying the annealing separator according to claim 3 onto the surface of the silicon dioxide film, and annealing the steel sheet.
 5. An annealing separator comprising the magnesium oxide for an annealing separator according to claim
 2. 6. A method for manufacturing a grain-oriented electromagnetic steel sheet, comprising: the step of forming a silicon dioxide film on a steel sheet surface; and the step of forming a forsterite film on the steel sheet surface by applying the annealing separator according to claim 5 onto the surface of the silicon dioxide film, and annealing the steel sheet. 